The area you draw on is painted over and the image is re-encoded. What was underneath is no longer in the file, unlike a black rectangle drawn in an editor, which comes off in one click.
What it does not do: Redaction covers what you have drawn: nothing is detected automatically, no face is found for you, because that would need a model downloaded from a third party. Check the result before publishing: an area you forgot stays readable. The image is re-encoded, so slightly recompressed; keep the original.
Nothing is uploaded. Cut your network connection once this page has loaded: the tool keeps working. How to check it yourself.